A considerable number of physical effects are known which allow determination of temperature of an object of interest, and it has been found convenient to broadly classify temperature sensing methods and devices depending on whether a sensor element is in physical contact with an object or else is separated from the object of interest.
These broad classes of contact and non-contact methods, in turn, have been subdivided further; in particular, with respect to contact methods, subdivisions include methods based on the thermal expansion of gases, liquids, or solids, on the variation of electrical resistance of substances as a function of temperature, and on thermoelectric effects. In the following, particular attention is paid to methods based on the variability of electrical resistivity as a function of temperature.
Popular among devices based on such variability are thermometers in which a variable-resistance conductor element is made of nickel or platinum; determination of the electrical resistance of a such element can be carried out on the basis of Ohm's law and may involve the use of a Wheatstone bridge.